The intelligence services fear that Britain could be subject to a Beslan-style siege, with multiple hostages forced to plead for their lives on camera.

Whitehall sources have said that the threat is considered so credible that MI5, the police and the SAS have conducted at least two mock counter-terrorism exercises to work out how to deal with such an eventuality.

The last exercise, shortly before Christmas, took place at an RAF base near Chester. Five police forces were involved in an operation that envisaged an international conference being stormed by terrorists, who then held a group of children hostage in a creche wired with explosives.

Operation Northern Synergy saw a number of police chiefs assume the Gold Command - ultimate responsibility for co-ordinating the response. The commanders liaised with the government's Cobra committee, which is activated during times of national crisis. In the scenario the terrorists were equipped with mobile phones and a satellite uplink that allowed them to beam pictures of the hostages on to television screens. The operation ended with a decision to send in the SAS.

'This scenario is something that is very much on the radar screen,' said one counter-terrorism source. 'We have envisaged a British Beslan for several years.' Beslan in south-west Russia was the scene of a horrific siege when on 1 September, 2004, 1,200 schoolchildren and adults were taken hostage by Muslim terrorists. The siege resulted in the security services storming the school and the deaths of 344 of the hostages.

Growing fears that domestic terrorists could seize hostages in Britain and parade them on television and websites were underlined last week when police conducted a series of raids across the West Midlands.

Nine men were arrested under suspicion of attempting to kidnap a Muslim soldier. There were claims last week that the alleged plotters intended to film the soldier pleading for his life and then behead him, but security sources say that they cannot confirm the details.

However, it is thought they are studying the similarities with an alleged plot in Canada that was disrupted last year. Last May the Royal Canadian Mounted Police (RCMP) arrested 17 men who were allegedly planning to storm the country's parliament, take hostages and behead the Prime Minister, Stephen Harper. It is known that those behind the alleged plot in Canada had links with terror suspects in Britain.

'We know from the plot in Canada that terror cells have been considering plots to kidnap high-profile individuals,' a senior counter-terrorism official said. 'We have to be alert to the possibility that Islamic extremist groups may be considering many forms of attack, including kidnappings or taking hostages on a large scale. Dame Eliza [Manningham-Buller, the head of MI5] has always made it clear that the threat we face comes in many different forms.'

A spokesman for the RCMP declined to comment on whether it was liaising with its counterparts in Britain. 'We cannot comment on operational matters,' the spokesman said.

Birmingham's Muslim community held a meeting last night to discuss the aftermath of the police raids, amid growing criticism of the way in which the media reported the case.

On Thursday, West Midlands police were given a further seven days to hold the men after applying for an extension at Coventry magistrates' court.

Assistant Chief Constable David Shaw said that a number of procedures had to be completed before formal interviews could take place. 'Far better that we use a measured, calculated, meticulous response than rush into things,' said Shaw, acknowledging that he was aware of the police intrusion into the lives of local people. We do need to balance the disruptions we are causing you with doing a thorough and professional job,' Shaw said.

Forensic specialists have been scouring 15 properties in Birmingham that were seized during Wednesday's operation.

A further three search warrants were executed on Friday. Police said six searches had been completed and a 'significant quantity' of exhibits had been gathered.